Method for joining tubular parts of metal by forge/diffusion welding

ABSTRACT

A method for joining thin-walled tubular parts (1, 2) of metal by forge/diffusion welding comprising establishing a joint (3) between opposed bounding surfaces (4, 5), the joint having increasing height from one side (7) to the other. During heating of the parts the joint (3) is supplied with an inert or reducing flushing gas, for instance hydrogen, until the oxides are removed from the joint surfaces (4, 5). By subsequently pressing the parts together, complete forge/diffusion welding is obtained. It is also suggested to reduce the cross-section of the parts excentrically in the joint area and press the parts out to their initial cross-section during the forge/diffusion welding. A packer (8) is used to close the joint (3) on the side where the distance between the parts (1, 2) is the greatest. Means (10, 11) are shown for performing the welding under water.

The present invention relates to a method for joining tubular parts ofmetal by forge/diffusion welding, comprising the following steps:

establishing a joint between opposed bounding surfaces on the respectiveparts to be joined, which joint having the form of a cavity with varyingheight,

heating the parts to a predetermined temperature at least in those areasbordering on the joint, the cavity of the joint during the heating beingsupplied with an inert or reducing flushing gas,

joining the parts by pressing them together, and cooling the parts.

Such a method is described in the Applicant's International PublicationNo. NO. 83/03565, published on 27th October 1983. In this method thejoint is initially given the form of a cavity having increasing heightfrom the periphery towards the middle, i.e. in case of tubular parts thejoint would be approximately closed along the periphery both on theinside and the outside of the parts. The joint is provided with at leastone connecting conduit for the supply of inert or reducing flushing gas.

The object of the present invention is to simplify and improve the abovemethod, e.g. as regards the flushing and the initial form of the cavity.

This is obtained according to the invention by a method of the abovetype, which is characterized in that the joint during the heating andflushing is held sufficiently open to permit flushing transversallythrough the joint, the gas being supplied from the outside of the joint.

This simplifies the supply of flushing gas and also ensures quick andefficient flushing of the joint.

According to a further feature of the invention, the joint is initiallygiven increasing height from one side of the tubular parts to the other.This will simplify the preceding machining of the ends of the tubularparts, for instance by giving them the form of truncated cones havingdifferent apex angles. Such shaping of the ends of the tubular partspermits the use of the method with less restrictive requirements to theroundness of the tubular parts.

In accordance with an advantageous embodiment of the invention, thecavity of the joint may be closed on one side by means of a sealingdevice comprising means for supply of the flushing gas.

The method according to the invention can also be adapted for weldingunder water. In this case it will be advantageous to cover the jointarea on the opposite side of the sealing device by means of a collar orthe like, which is filled with flushing gas through the joint in orderto prevent direct contact with the surrounding medium. Before commencingthe heating, the space between the sealing device and the joint area maybe emptied for surrounding medium by displacing it by means of flushinggas.

According to a further advatageous feature of the invention, during thepressing together of the parts the joint is brought to closesuccessively in the direction towards the supply direction of theflushing gas. Thereby the flushing gas will be forced out of the jointduring the joining, so that it will not be necessary to evacuate thejoint in advance.

It has surprisingly been found that the joining of the parts can takeplace very quickly, 1-2 sec. without post-heating, if one as reducinggas is using hydrogen with less than about 100 ppm of H₂ O/O₂ for usualstructural steels, e.g. steels containing about 0,2 percent C, 0,7percent Mn and 0,3 percent Si. For high strength steels alloyed withe.g. 0,2 percent C, 1,6 percent Mn, 0,55 percent Si and residues of Tior Al, the joining can also take place very quickly if the hydrogen hasless than about 15 ppm of H₂ O/O₂. In order to ensure complete oxideremoval from the joint for alloyed steels, it is suggested according tothe invention that the flushing with hydrogen is maintained until thecontents of H₂ O/O₂ in the flushing gas leaving the joint is less thanabout 15 ppm.

For the better understanding of the invention it will be described moreclosely with reference to the examplifying embodiments shown in theappended drawings, in which:

FIG. 1 shows a section through two juxtaposed tubular parts beforewelding;

FIG. 2 shows at a larger scale a fraction of FIG. 1 immediately beforewelding;

FIG. 3 shows the parts in FIG. 2 after completed joining.

The two parts 1, 2 are constituted by relatively thin-walled pipes whichare to be welded together at a joint 3. In the joint area the tubularparts are internally provided with a sealing device 8 in the form of aninflatable packer so that the area around the joint 3 can be isolated onthe inside of the tubular parts. Flushing gas is supplied through aconduit 6 in the packer.

The embodiment shown in FIG. 1 concerns welding of two pipes underwater. The pipes are arranged vertically with the packer 8 on theinside, while an induction coil 9 is arranged around the joint 3 on theoutside of the pipes for heating the joint area. In order to preventwater from coming into direct contact with the joint area during thewelding, the joint area, on the opposite side of the packer 8, iscovered by a collar 10, which advantageously may consist of heatresistant material, for instance asbestos-containing material. At thetop the collar 10 is pressed sealingly against the pipe 1 by means of asealing ring 11.

From FIG. 1 it may also be seen that the sealing device 8 is providedwith a conduit 12 which leads to the surroundings from the space definedbetween the sealing device and the pipes. This conduit may be opened andclosed by means of a valve 13. Furthermore, the sealing device 8 isprovided with a supply conduit 14 for inflating medium, which may be anysuitable fluid, for instance water.

When carrying out this example of the method according to the invention,the various parts are first brought in position as shown in FIG. 1, andthe sealing device 8 is subjected to a suitable pressure. Thereafter,flushing gas is supplied through the conduit 6 while the valve 13concurrently is held in the open position, so that any water in thespace between the sealing device and the tubular parts is displaced outthrough the conduit 12. When almost all the water has been displaced,the valve 13 is closed, and flushing gas will now flow out through thejoint 3 and displace any water between the collar 10 and the joint area.

As flushing gas an inert gas may be used, but in many instances it willbe possible to use a reducing gas, for instance hydrogen, already fromthe start. For many types of steel it is not necessary to evacuate thejoint area for hydrogen after the heating and welding process havebegun, particularly if the joint is given such a form that it will closesuccessively in the direction towards the supply direction of theflushing gas when the joint is pressed together.

Such an advantageous joint form is schematically suggested in FIG. 2,this figure corresponding to a portion at a larger scale of the jointarea to the right in FIG. 1, however with the difference that thewelding takes place in air, as it is suggested with the burning hydrogenflowing out of the joint on the outside of the tubular parts. Here thejoint 3 has the form of a wedge. The arrows F indicate a variable forcefor pressing the parts together, while the bellshaped curve to the leftof the parts 1 and 2 indicates the axial temperature distribution inthese parts.

Following sufficient flushing of the joint 3 and heating of the parts 1,2, the parts are pressed together as suggested by the arrows F. Thejoint 3 will first close at 7, and by further pressing together thelimit line for the contact between the surfaces 4 and 5 will movesuccessively towards the left so that no pockets will form in the joint.The gas in the joint is therefore pressed out even if the gas should beat a relatively high pressure.

It will be seen that the parts 1, 2 are given reduced cross-section inthe joint area. This is done in order to more easily obtain a tri-axialstress condition in the joint area when the material, during thepressing together of the parts, flows so that these obtain the formshown in FIG. 3.

From FIG. 2 it may be seen that the narrowest portions of the parts 1, 2lie somewhat eccentrically with respect to the middle line of the pipewalls. This eccentricity is adapted to the form of the joint and the wayit will make the material flow during the pressing together, so that theend result will be generally symmetrical about the middle line, as shownin FIG. 3.

It will be understood that many different types of sealing devices 8 maybe used. If the pipes are so thin-walled in relation to their diameterthat the joint surfaces 4, 5 will give insufficient alignment andguidance between the tubular parts during the joining, the sealingdevice 8 may be formed to give the necessary axial guidance.

For flushing the joint 3 a number of different gases may be used,particularly reducing gases. However, hydrogen has so far proved to bethe most suitable for the steel types usually used in offshorepipelines. Furthermore, one has found that if one uses sufficiently purehydrogen and a material temperature of 1000° C. to 1250° C., dependingupon the steel quality, one may obtain almost immediate reduction of allsurface oxides. Since the joint surfaces thus become entirely clean, thepressing together of the parts may take place very quickly, i.e. like inforge welding. In using the method according to the invention forwelding common commercial steels, the hydrogen should contain less than100 ppm H₂ O/O₂. For high strength steels alloyed with Mn and Si andcontaining residues of Ti or Al, the contents of H₂ O/O₂ should be lessthan 15 ppm. A check on whether the joint has become sufficiently cleanbefore the pressing together may be obtained by measuring the contentsof H₂ O/O₂ in the hydrogen flowing out of the joint during the flushing.The flushing should be continued until the contents of the impuritieshave sunk to for instance 15 ppm.

Even though FIG. 1 shows the use of an induction coil 9 for heating theparts, it will be clear to the skilled person that a number of otherheating methods may be used, for instance high frequency resistanceheating, which is advantageous at wall thicknesses greater than about 12mm.

What is claimed is:
 1. An improved method for joining one tubular metal part to another tubular metal part by forge/diffusion welding, said metal parts being elongated and having a generally constant first cross section, comprising the steps:establishing a joint between opposed bounding surfaces on said one and said other parts, said opposed surfaces defining a cavity therebetween before welding, the peripheries of said opposed bounding surfaces being in close proximity to each other at an outer side of said parts and further apart at the other, inner side of said parts, said parts each having a reduced cross section in the area of said joint, before welding; heating said one and said other parts to a predetermined temperature at least in areas bordering said joint; supplying a reducing flushing gas to said cavity during said heating, said flushing gas being introduced into the cavity at said other, inner side of said parts and flowing transversely through said cavity; welding said parts to each other by pressing said parts together, with a predetermined speed, until said areas having initially reduced cross sections have a second cross section, after welding, substantially equal to said generally constant first cross section of said elongated parts; and cooling the parts.
 2. The method according to claim 1, wherein the joint cavity initially has gradually increasig height from the outer side of the tubular parts to the inner.
 3. The method according to claim 1, wherein the cavity of the joint is closed on one side by means of a sealing device including means for supplying the flushing gas.
 4. The method according to claim 3, wherein the joint area, on the opposite side of the cavity from the sealing device, is covered by a collar or the like , the collar filled with flushing gas through the joint in order to prevent direct contact with the surrounding medium.
 5. The method according to claim 3, wherein before the heating step, the surrounding medium is emptied from the space between the sealing device (8) and the joint area by displacing the medium by means of flushing gas.
 6. The method according to claim 1, wherein during the pressing together of the metal parts, the joint closes in the direction towards the supply direction for the flushing gas.
 7. The method according to claim 6, wherein hydrogen having less than 100 ppm H₂ O/O₂ is used as reducing gas.
 8. The method according to claim 7, wherein hydrogen having less than 15 ppm H₂ O/O₂ is used as reducing gas.
 9. The method according to claim 8, wherein flushing with hydrogen is maintained until the contents of H₂ O/O₂ in the flushing gas flowing out of the joint (3) is less than about 15 ppm.
 10. The method according to claim 2, wherein the cavity of the joint is closed on one side by means of a sealing device including means for supplying the flushing gas.
 11. The method according to claim 4, wherein before the heating, the surrounding medium is emptied from the space between the sealing device and the joint area by displacing the medium by means of flushing gas. 